The SessionHandler class

(PHP 5 >= 5.4.0, PHP 7)

Einführung

SessionHandler is a special class that can be used
to expose the current internal PHP session save handler by inheritance.
There are seven methods which wrap the seven internal session save handler
callbacks (open, close,
read, write,
destroy, gc and
create_sid). By default, this class will wrap
whatever internal save handler is set as defined by the
session.save_handler
configuration directive which is usually files by
default. Other internal session save handlers are provided by PHP
extensions such as SQLite (as sqlite), Memcache (as
memcache), and Memcached (as
memcached).

When a plain instance of SessionHandler is set as the save handler using
session_set_save_handler() it will wrap the current save handlers.
A class extending from SessionHandler allows you to override
the methods or intercept or filter them by calls the parent class methods which ultimately wrap
the interal PHP session handlers.

This allows you, for example, to intercept the read and write
methods to encrypt/decrypt the session data and then pass the result to and from the parent class.
Alternatively one might chose to entirely override a method like the garbage collection callback
gc.

Because the SessionHandler wraps the current internal save handler
methods, the above example of encryption can be applied to any internal save handler without
having to know the internals of the handlers.

Please note the callback methods of this class are designed to be called internally by
PHP and are not meant to be called from user-space code. The return values are equally processed internally
by PHP. For more information on the session workflow, please refer session_set_save_handler().

This class is designed to expose the current internal PHP session save handler, if you want to
write your own custom save handlers, please implement the SessionHandlerInterface
interface instead of extending from SessionHandler.

// we'll intercept the native 'files' handler, but will equally work// with other internal native handlers like 'sqlite', 'memcache' or 'memcached'// which are provided by PHP extensions.ini_set('session.save_handler', 'files');

Since this class' methods are designed to be called internally by PHP as part of the normal session workflow,
child class calls to parent methods (i.e. the actual internal native handlers) will return FALSE unless
the session has actually been started (either automatically, or by explicit session_start().
This is important to consider when writing unit tests where the class methods might be invoked manually.

User Contributed Notes 2 notes

As the life-cycle of a session handler is fairly complex, I found it difficult to understand when explained using just words - so I traced the function calls made to a custom SessionHandler, and created this overview of precisely what happens when you call various session methods:

Here is a wrapper to log in a file each session's operations. Useful to investigate sessions locks (which prevent PHP to serve simultaneous requests for a same client).Just change the file name at the end to dump logs where you want.